A large body of evidence now exists to support the fact that the zygopophysial joints (facet joints) can be pain-producing structures. In particular, has been shown that the facet joints can be a source of chronic spinal pain in the cervical, thoracic and lumbosacral regions. This pain, which can be due to trauma to and/or degeneration of the facet joint, can be disabling in some patients.
Anatomical dissections reveal that the facet joint is innervated by the articular branches of the medial branch of the spinal nerve. Lesioning this nerve has been shown to relieve pain, but regrowth of the nerve is inevitable and pain returns.
Electrical stimulators of other neural structures has been proposed to provide pain relief. U.S. Pat. No. 5,041,974 to Walker et al, entitled MULTICHANNEL STIMULATOR FOR TUNE STIMULATION, includes a user interface enabling the selection of a channel and the creation of a stimulus wave signal. A slave circuit associated with the channel receives the wave building signal and generates a corresponding low-power stimulus. An output circuit coupled to the slave electronically isolates the stimulus from the other channels, amplifying and converting it to produce a high-fidelity stimulus wave signal.
There have also been developed neuro-type stimulators which are responsive to patient conditions, and adjust their function in accordance therewith. For example, in U.S. Pat. No. 5,031,618 to Mullett, entitled POSITION-RESPONSIVE NEURO STIMULATOR, a position sensor such as a mercury switch which may be used to determine whether a patient is erect or supine, is implanted in the patient. This position information is then used to vary stimulation intensity, in terms of pulse amplitude, pulse width, and a number of pulses per second and other factors. The output of the pulse generator is applied to the spinal cord, peripheral nerves and/or targets in the brain with leads in electrodes in a manner consistent with a given medical need.
To applicant's knowledge, however, no such neuro stimulators, whether adaptive or fixed in their operation, have been applied to the specific problem of pain relief relative to the zygopophysal or facet joints. As the '618 patent points out, stimulation of this type has so far been limited to the treatment of chronic intractable pain requiring spinal cord depolarization. Accordingly, one object of this invention is to provide novel apparatus specifically designed to block painful impulses transmitted by the medial branch of the spinal nerve.
Another important object of this invention is to provide such apparatus in the form of an electrical nerve stimulator that is implantable.
It is a related object of this invention to create a pain-relief mechanism and accompanying methodology that is long lasting while being minimally invasive so as to reduce medical complications.